It is common practice after workpieces have been produced to inspect them on a positioning apparatus such as a coordinate measuring machine (CMM) or other type of coordinate positioning apparatus. Such coordinate positioning apparatus typically have a quill onto which a contact probe, such as a touch trigger probe or scanning probe, is mounted such that the contact probe can be driven in three orthogonal directions X, Y, Z within a working volume of the machine.
U.S. Pat. No. 5,189,806 describes an articulating probe head capable of orientating a contact probe with two rotational degrees of freedom to enable the probe to be used in an operation for measuring the surface of workpieces. In general, such a probe head includes two rotary drive mechanisms which enable a contact probe to be orientated about two substantially orthogonal rotary axes. Such an articulating probe head may be mounted on the quill of a CMM to enable the probe to be positioned in 5 degrees of freedom (i.e. 3 linear degrees of freedom provided by the CMM and 2 rotary degrees of freedom provided by the articulating probe head).
The contact probe has a contact element, such as a stylus tip, for contacting surfaces of the workpiece to cause deflection of a stylus of the probe, the probe generating signals in response to the deflection. A stylus tip is usually calibrated before the probe is used to measure a workpiece. It is known to calibrate the stylus tip diameter by measuring a calibrated sphere (so called Master Ball) with the probe. This measurement may be carried out at different orientations of the probe, for example as disclosed in WO00/25087 or WO2011/002501. However, a disadvantage with such a method is that it is reliant on the accuracy of the coordinate positioning machine and/or probe head and is time consuming.